Insulin medications were originally produced from animals (cows, pigs, and salmon). They worked well but some patients experienced undesirable reactions. So synthetic insulin was produced. Synthetic insulin is produced in a laboratory, using the E. coli bacteria, and is supposed to be similar to the natural insulin produced by the human body.
Fewer and fewer companies are now making animal insulin available, switching instead to "human" or synthetic insulin. However there have been several claims or reports of adverse reactions to synthetic insulin. Some of these reactions include low blood sugar, confusion, memory loss, seizures, coma, and even death.